The present invention relates to cell selection in a cellular mobile communications system, and in particular to fast cell site selection during soft handover.
In a cellular mobile communications system, each base station has associated with it a cell covering a certain area (“footprint”). A mobile unit within the coverage area of the cell communicates with the system by transmitting radio signals to, and receiving radio signals from, the base station associated with the cell. The shapes and sizes of different cells can be different and may vary over time. The respective coverage areas of adjacent cells generally overlap with one another so that at any given time, a mobile unit may be capable of communicating with more than one base station.
Within each cell, it is necessary for the base station to transmit to each wanted user (i.e. each active mobile unit) in a multi-user and multi-path environment. In order to achieve satisfactory signal detection at low bit error rates, the multi-user interference (MUI), sometimes referred to also as multiple access interference (MAI), must be reduced to an acceptable level. The amount of interference that is experienced in a cell is dependent on the loading in that cell, which is dependent on the number of users and the data rates of those users. The amount of interference generally increases non-linearly as the loading increases. Typically, a threshold is set which specifies the maximum loading in a cell.
At various times, for example during call setup, network acquisition or handover between cells, it is necessary for a mobile unit to perform a cell selection operation, i.e. to select a base station with which to communicate. In known cell selection techniques, the decision as to which cell to use is based on a measure of signal quality within the cells. For example, a mobile unit wishing to set up a call, or to acquire the network, or to perform handover (either hard or soft handover) may measure the qualities of signals received from various base stations and use those measures to select a base station with which to communicate.
In a technique known as Fast Cell Site Selection (FCSS), a mobile unit which is in a soft handover region (i.e. in communication with two or more base stations) selects a single base station with which data communication is to take place every radio frame. The selection is based on the quality of the signals received from the active base stations. The mobile unit transmits the identification number of the selected base station to all of the base stations with which it is in communication. Each of the base stations receives the identification number, and determines whether or not it is the selected base station. The selected base station then transmits data to the mobile unit in the next radio frame, while the other base stations suppress data transmission to that mobile unit in that radio frame.
FCSS can improve the overall performance of a cellular communications system by reducing the number of channels that are transmitted and thus reducing the interference to other users. The technique can also prevent communication from taking place on a channel which has faded. Since the base stations themselves determine whether or not to transmit data (rather than the network), cell selection can be carried out quickly, and thus the system can respond quickly to changes in environmental conditions.
A problem in known cellular communications systems is that, if the selected cell is heavily loaded, then when the mobile unit starts communication with that cell, an undesirably large increase in interference may occur. This is due to the fact that interference increases non-linearly with cell loading. Furthermore, by adding a further mobile unit to a cell which is already heavily loaded, there is a risk that the cell may reach its loading threshold. This may result in, for example, a dropped connection, some users being barred from that cell, the data rates of some users being reduced, or the latency of the system being increased due to buffering of packets. The above problems are particularly compounded where high data rate communication takes place between the selected base station and the mobile unit.
Another problem in cellular communications systems is that loading thresholds are usually set to levels somewhat below the maximum capacities of the cells. This is to allow some reserve capacity, so that, if a new mobile unit enters a cell, the reserve capacity may be used (by temporarily exceeding the loading threshold) to prevent that mobile unit from being barred from the cell. However, setting the loading threshold to a low level may result in less efficient usage of cell capacity in comparison to the case where a higher threshold is set.
A further problem which may arise in particular in high data rate cellular communications systems is that loading levels may change rapidly and it may be difficult for the system to respond quickly to the changes.
It is therefore desirable to provide a system in which some or all of the above problems are reduced.
In International patent publication number WO99/59366 in the name of Fujitsu Limited, the entire subject matter of which is incorporated herein by reference, a technique is disclosed in which, when a mobile unit is capable of transmitting with two or more base stations, a base station for transmission is selected on the basis of which base station would cause the least interference. This technique is effective in reducing interference levels in the system, but it does not address the problems associated with loading thresholds discussed above.
Reference is also made to International patent publication number WO99/59367 in the name of Fujitsu Limited, the entire subject matter of which is incorporated herein by reference, which contains background material of relevance to the present invention.